Thermal spraying is a process of applying coatings of high performance materials, such as metals, alloys, ceramics, cermets and carbides, onto more easily worked and cheaper base materials. The purpose of the coating is to provide enhanced surface properties to the cheaper bulk material of which the part is made. Because of its ability to deposit virtually any material (and many combinations of materials) thermal spray has a wide and growing range of applications.
Coatings are a pervasive technology, permeating throughout all of industry and high technology applications. Coating technology is an enhancing technology that improves products and reduces cost. In many applications, coatings make it possible to achieve ends that cannot be achieved in any other known way, or at least in any way that is affordable. Examples of applications in different technologies are shown in Table 1.
Coatings are most easily grouped according to their primary function, as listed in Table 2. However, a given coating can often provide more than one of these basic functions. Some important applications are for thermal insulation, wear resistance, corrosion and chemical resistance, abradable and abrasive coatings, electrically conductive or resistive coatings, medically compatible coatings, dimensional restorative coatings, and polymer coatings. Recently, there has been strong interest in net shape spray forming techniques.
Specific applications range from thermal barrier coatings in gas turbines, to protective coatings for rocket nozzles, to internal combustion engine components such as piston rings, to newer applications in the prosthetic device area, to hardfacing load bearing surfaces, to anti-corrosion coatings on bridges and other infrastructure, to literally hundreds of others. Fabrication of ceramic substrates for electronic circuits by thermal spray techniques has long been considered, but is not yet widely accepted. This situation is expected to improve with the availability of improved spray processes which provide denser coatings with more uniform (homogeneous) and more resistive coatings. Table 1 lists many of the existing application areas for thermal spray coatings.
Dimensional restoration of worn landing gear assemblies is a potential market for advanced thermal spray technology. Currently, the quality of available thermal spray coatings is not sufficient for the FAA to approve the process for restoration of these and other critical aircraft assemblies. The potential to achieve true metallurgical bonding at high velocity holds the promise of acceptance of thermal spray repairs for these assemblies, opening up a large new market that does not now exist.
Thermal spray includes a variety of approaches, but can be grouped into three main coating processes: combustion, wire-arc, and plasma. Each particular approach has advantages and disadvantages that tend to position it in a particular area or areas of application. These include (in roughly ascending order of coating quality and with particle impact velocities listed in parenthesis): flame spray (30 m/s), flame wire spray (180 m/s), wire-arc spray (240 m/s), conventional plasma spray (240 m/s), detonation gun (910-1200 m/s), high-velocity oxyfuel (HVOF) (610-1200 m/s), high-energy plasma (240-1220 m/s) and vacuum plasma (240-610 m/s).
Thermal spray techniques can be further subdivided into continuous and detonation processes. Most approaches involve continuous processes. The detonation gun, marketed for years by Union Carbide under the trade name D-Gun.RTM. and more recently by Praxair, is the most notable exception. A recent variant on the D-Gun.RTM. is the DEMETON gun being marketed by a Russian firm in the U.S. Recent improvements in HVOF spray make it competitive with D-Gun.RTM. applied coatings in some applications. HVOF coating quality is roughly comparable to that of the D-Gun.RTM..
Existing thermal spray processes are compared in Table 3.
Thermal spray has a rich history, but there is considerable room for improvement in the technology. There are substantial limitations in present thermal spray technology which have slowed or prevented expansion of existing markets and penetration into new markets and new application areas. The quality of coatings produced by present thermal spray technology and its economic viability is limited by a variety of factors including the following:
A need exists for higher particle impact velocity, which generally produces better coatings. PA1 More uniformity of spray pattern is needed. More uniform spatial & temporal velocity distribution desired. PA1 Sensitivity to feedstock materials and other process variables are experienced. PA1 More efficient use of the energy is needed to melt coating materials. PA1 Deposition efficiencies are less than 50% for some materials. PA1 Coating properties are not yet equivalent to those of wrought material. PA1 Cost of newer high-performance materials is relatively high. PA1 Improved coating consistency & reproducibility are needed. PA1 More reliable equipment is needed. PA1 Higher spray rates are generally desirable. PA1 No industry standards exist for spray guns, and few for coating materials. No coating can be considered generic and reproducible at this time, partly due to the inability to accurately model existing systems.
High quality coatings are "generally" characterized by high adhesion and cohesion strengths, low porosity, low oxide inclusions (except for some cases where the phases are small and well dispersed), high hardness, and other properties designed for specific applications such as electrical or magnetic properties, or machinability for finishing.
Particle impact velocity is one of the most important factors in coating quality. One of the main areas of research and innovation in the industry has been the quest for ever higher velocities. Higher velocity impact generally produces denser, harder, and more uniform coatings with less porosity and with higher adhesion and cohesion. Porosity is the largest source of coating failure and is usually indicative of poor coating cohesion and a high degree of unmelted or cold-particle entrapment. High velocity impact forces splats to fill in voids, and the kinetic energy which is converted to heat during the impact reduces the number of unmelted particles, which reduces porosity. Oblique spraying, off perpendicular, should be significantly improved by high velocity, through reduction of shadow porosity effects. In addition, higher velocity tends to produce coatings with less induced stresses.
Ideal characteristics of the plasma spray process are uniform and controllable velocity of particles on impact, sufficient velocity to produce a high density deposit without "exploding" the molten or partially molten droplets on impact, uniform and controllable heating of particles, attainment of fully molten or plastic particles without vaporization or undesired reactions, isolation from or controlled interaction with the ambient environment, and stable process conditions with highly reproducible results.
Despite the limitations of present thermal spray systems, a $2B market has developed over the years. Penetration of thermal spray technology into new application areas, such as the automotive industry and electronics is, however, fundamentally constrained by the limitations in the existing technology as described above. The highest quality coatings are also the most difficult to make due to the high temperatures required to melt the powder materials and the high density of the powders such as the refractory metal alloys. Existing thermal spray technologies must work very hard to achieve the velocities now being produced. A contributing factor is the difficulty of controlling the chemical environment and generally preventing oxidation reactions from occurring on the surface of the powder particles prior to impact on the substrate. All of these factors effect coating quality. New applications and expanded markets for existing applications are expected to occur in the industry if dramatic improvements in performance of the technology are introduced and if costs can be reduced significantly.
Wire-arc spraying is a well established segment of the thermal spray market which is used primarily for producing corrosion and wear protection coatings. It is one of the most efficient methods of producing thick coatings. In the two-wire arc process, two insulated metallic wire electrodes are continuously fed to an arc point where a continuously flowing gas stream is used to atomize and spray the molten electrode material in the arc. Some configurations utilize a single feed wire and a non-consumable electrode.
Wire-arc spraying has several advantages over other thermal spray techniques. It produces a high spray rate. The process is energy efficient since all input energy is used to melt the metal. Energy efficiency also means typically lower substrate temperatures. Wire-arc spraying is generally simpler to operate. Equipment is relatively low in cost; and safety is enhanced by eliminating combustible gases. These advantages are counter-balanced by the fact that only non-brittle metallic wires may be used, which generally limits the materials that can be sprayed. In addition, atmospheric operation produces considerable noise and UV and infrared radiation which introduce safety hazards. The use of cored wires allows the introduction of alloys and other materials such as ceramics into the spray. Characteristic arc spraying parameters are shown in Table 4.
The metal droplets are already molten when they come off the wire tips, and so the primary function of the gas flow is to simply atomize and accelerate the molten droplets towards the substrate. However, the air flow begins cooling the molten droplets immediately after leaving the arc zone. Thus wire-arc spray devices attempt to minimize the flight time to reduce this cooling effect in order to ensure the particles are still molten on impact. This places an additional limitation on the velocity the particles can attain. The use of air to accelerate the molten droplets can also lead to undesirable surface oxidation. Needs exist for improved thermal coating apparatus methods and products.